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The Heat and Buried Cable Conundrum

One of the most important factors affecting underground cable ampacity is the thermal resistivity of the soil. A soil’s thermal resistivity varies with moisture content; furthermore, heat generated by cables can cause soil to dry. The ability of the soil to maintain its thermal resistivity in the presence of a heat source is known as thermal stability. Soil will increase in resistivity due to drying caused by heating from underground sources. Because soil is not thermally stable, accounting for its fluctuating thermal resistivity makes cable ampacity calculations difficult. This paper will examine the information available from standard soil tests and the information these tests may provide relating to the migration of moisture in soil and the resulting changes in soil resistivity. Furthermore a method is suggested for including this information in underground cable ampacity calculations.

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